Radio communication system, radio station, radio terminal, communication control method, and computer-readable medium

ABSTRACT

A radio network ( 6 ) and a radio terminal ( 4 ) are configured to, when the radio terminal ( 4 ) is using a first cell ( 10 ) served by a first radio station ( 1 ) as a primary cell and a second cell ( 20 ) served by a second radio station ( 2 ) as a secondary cell, change the primary cell from the first cell ( 10 ) to a third cell ( 30 ) served by a third radio station ( 3 ) while keeping communication status information regarding the radio terminal ( 1 ) on the second cell ( 20 ). It is thus, for example, possible to enable a communication service in the secondary cell to be continued even after the primary cell is changed when the primary cell is changed during execution of carrier aggregation (e.g., Inter-eNB CA) on a plurality of cells served by different radio stations.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 16/452,014 filed on Jun. 25, 2019, which iscontinuation application of U.S. patent application Ser. No. 14/655,984file on Jun. 26, 2015, which is issued as U.S. Pat. No. 10,375,606,which is a National Stage Entry of international applicationPCT/JP2013/006804, filed on Nov. 20, 2013, which claims the benefit ofpriority from Japanese Patent Application 2012-288211 filed on Dec. 28,2012, the disclosures of all of which are incorporated in their entiretyby reference herein.

TECHNICAL FIELD

The present invention relates to a radio communication system in which aradio station and a radio terminal communicate with each other using aplurality of cells.

BACKGROUND ART

In order to improve communication quality and speed further to cope withan abrupt increase in mobile data traffic in recent years,standardization of a carrier aggregation (CA) function of allowing aradio base station (eNode B (eNB)) and a radio terminal (user equipment(UE)) to communicate with each other using a plurality of cells has beendiscussed in 3GPP LTE (Long Term Evolution). The cells that a UE can usein the CA are limited to cells of one eNB (i.e., cells served by oneeNB).

The cells used by the UE are classified into a primary cell (PCell)which has already been used as a serving cell at the start of CA and asecondary cell (SCell) which is used additionally or subordinately.Non-Access Stratum (NAS) mobility information, security information(security input), and the like are sent and received through the PCellduring radio connection (re)-establishment (RRC connectionEstablishment/Re-establishment) (see Non-Patent Literature 1). A DLcarrier corresponding to the PCell is a DL primary component carrier (DLPCC) and its corresponding UL carrier is an UL PCC. Similarly, a DLcarrier corresponding to the SCell is a DL secondary component carrier(DL SCC) and its corresponding UL carrier is an UL SCC.

When a UE that has been executing CA performs an inter-base stationhandover (Inter-eNB handover), the SCell(s) is released (see Non-PatentLiteratures 2 and 3). In the following, with reference to a sequencediagram of FIG. 24, a procedure in which a UE that has been executing CAon Cell1 and Cell2 of a source eNB1 performs a handover to Cell3 of atarget eNB2 will be described.

In step S1, a UE performs CA using Cell1 of eNB1 as a PCell and usingCell2 of the same eNB1 as a SCell. In step S2, the UE transmits aterminal measurement report (Measurement report) to the eNB1. In thisexample, the measurement report includes measurement results of Cell3that serves as a handover target.

In step S3, the eNB1 sends a handover request (handover (HO) preparationrequest) to the eNB2. The HO preparation request includes a list(sourceSCellConfigList) of SCell(s) (in this example, the Cell2) whichis used for CA in the eNB1. The HO preparation request may include alist (candidateCellInfoList) of candidate cell(s) used after thehandover. In step S4, the eNB2 sends to the eNB1 an acknowledgementresponse (HO preparation request ACK) to the handover request. The HOpreparation request ACK includes a list (sCellToReleaseList) of releasedSCell(s) (in this example, the Cell2). The HO preparation request ACKmay include a list (sCellToAddModList) of SCell(s) (in this example,Cell4) to be used after the handover.

In step S5, the eNB1 transmits a message (RRC ConnectionReconfiguration) instructing the UE to perform a handover to the Cell3of the eNB2. At this time, the sCellToReleaseList, thesCellToAddModList, and the like, which has been sent from the eNB2 tothe eNB1, are also transmitted to the UE. In step S6, the eNB1 sends acommunication status of the UE to the eNB2 (SN status transfer). In stepS7, the UE releases the SCell (i.e., the Cell2) (SCell release). In stepS8, the UE synchronize with the eNB2 in the Cell3 and completes thehandover process (Synchronization and RRC Connection ReconfigurationComplete). In step S9, when the UE has been instructed to add the Cell4as SCell, the UE executes the request (SCell addition).

In step S10, the eNB2 sends a path switching request to a core network(EPC) (Path switch request). In step S11, the EPC executes pathswitching (Path switch). In step S12, the EPC notifies the eNB2 ofcompletion of path switching (Path switch request ACK). In step S13, theeNB2 notifies the eNB1 of the release (acknowledgement of release) ofterminal information (UE context release). In step S14, when the eNB2has instructed the UE to add the Cell4 as the SCell, the eNB2 instructsthe UE to activate the Cell4 (SCell activation). In step S15, the UEperforms CA using the Cell3 as the PCell and using the Cell4 as theSCell.

Moreover, a concept of Inter-eNB CA of aggregating a plurality of cellsserved by different eNBs has been proposed (Non-Patent Literature 4).For example, the Inter-eNB CA may use a macro cell served by a macrobase station (Macro eNB (MeNB)) and a pico cell served by a pico basestation (Pico eNB (PeNB)).

Further, a method has been proposed in which signals for control-planeincluding mobility management of a UE are transmitted and received usinga macro cell having a wide coverage and data-plane signals such as userdata are transmitted and received using a pico cell which providesrelatively better communication quality (Non-Patent Literature 5). Thismethod is referred to as C/U Split.

CITATION LIST Non Patent Literature

-   [Non-Patent Literature 1] 3GPP TS 36.300 V11.3.0, “Evolved Universal    Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial    Radio Access Network (E-UTRAN); Overall description; Stage 2,”    Section 7.5, September 2012-   [Non-Patent Literature 2] 3GPP TS 36.331 V11.1.0, “Evolved Universal    Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC);    Protocol specification,” Section 5.3.5.4, September 2012-   [Non-Patent Literature 3] 3GPP TS 36.423 V11.2.0, “Evolved Universal    Terrestrial Radio Access Network (E-UTRAN); X2 application protocol    (X2AP),” Section 9.1.1, September 2012-   [Non-Patent Literature 4] 3GPP RWS-120046, Samsung Electronics,    “Technologies for Rel-12 and Onwards,” 3GPP TSG RAN Workshop on    Rel-12 and Onwards Ljubljana, Slovenia, 11-12 Jun. 2012-   [Non-Patent Literature 5] 3GPP RWS-120010, NTT DOCOMO,    “Requirements, Candidate Solutions & Technology Roadmap for LTE    Rel-12 Onward,” 3GPP TSG RAN Workshop on Rel-12 and Onwards    Ljubljana, Slovenia, 11-12 Jun. 2012

SUMMARY OF INVENTION Technical Problem

In the Inter-eNB CA, for example, a cell of the macro base station(Macro eNB (MeNB)) and a cell of a low-power base station (Low-PowerNode (LPN)) are used simultaneously as a PCell and a SCell,respectively. In this case, bearers are separately configured in thePCell and the SCell. The UE communicates with the MeNB through a bearerin the PCell and communicates with the LPN through the bearer in theSCell.

The inventors of this application has conducted various studies on thecontinuity of communication services when a radio terminal (UE) that hasbeen executing Inter-eNB CA performs a handover and has found severalproblems. For example, a case in which a UE that is executing Inter-eNBCA using a cell of a MeNB as a PCell and using a cell of a LPN as aSCell performs a handover to another MeNB, and thus, a primary cell ischanged will be considered. When the handover procedure (FIG. 24) of aUE being executing normal CA (i.e., Intra-eNB CA) is applied to thishandover, the UE has to release the SCell (cell of the LPN). Thus, aproblem occurs in that communication services provided in the SCellcannot be continued after the handover. The cause of this problem is asfollows. In the case of Intra-eNB CA, the same bearer is configured inthe PCell and the SCell, a source eNB sends communication status (SNstatus) in the PCell to a target eNB, and a UE also keeps (stores) thecommunication status. Thus, in the case of Intra-eNB CA, the service ofthe SCell is continued after the handover based on the communicationstatus of the PCell. In contrast, when the SCell is released in theInter-eNB CA, the bearer information on the SCell, including thecommunication status of the SCell, is discarded. That is, the Inter-eNBCA does not have a mechanism for transferring the communication statusof the SCell to the target eNB, and thus the service of the SCell cannotbe continued after the handover.

Accordingly, an object of the present invention is to provide a radiocommunication system, a radio station, a radio terminal, a communicationcontrol method, and a program which contribute to enabling acommunication service in a secondary cell to be continued even after aprimary cell is changed, when the primary cell is changed duringexecution of carrier aggregation (e.g., Inter-eNB CA) on a plurality ofcells served by different radio stations.

Solution to Problem

In a first aspect, a radio communication system includes a radioterminal and a radio network including first to third radio stations.The first to third radio stations serve first to third cells,respectively. The radio terminal has a capability of, while using a cellof one radio station as a primary cell, using a cell of another radiostation as a secondary cell. Furthermore, the radio network and theradio terminal are configured to, when the radio terminal is using thefirst cell as the primary cell and the second cell as the secondarycell, change the primary cell from the first cell to the third cellwhile keeping communication status information regarding the radioterminal on the second cell.

In a second aspect, a first radio station includes a radio communicationunit that serves a first cell, and a communication control unit. Thecommunication control unit is configured to, when changing a primarycell from the first cell to a third cell served by a third radio stationwhile a radio terminal is using the first cell as the primary cell andusing a second cell served by a second radio station as a secondarycell, send to the second radio station at least one of (a) a request toabort or suspend communication with the radio terminal in the secondcell and (b) a request to report communication status informationregarding the radio terminal on the second cell.

In a third aspect, a second radio station includes a radio communicationunit that serves a second cell, and a communication control unit. Thecommunication control unit is configured to, when a primary cell ischanged from a first cell served by a first radio station to a thirdcell served by a third radio station while a radio terminal is using thefirst cell as the primary cell and using the second cell as a secondarycell, receive from the first radio station an instruction to abort orsuspend data communication with the radio terminal in the second cell,and abort or suspend the data communication.

In a fourth aspect, a third radio station includes a radio communicationunit that serves a third cell, and a communication control unit. Thecommunication control unit is configured to, when a primary cell ischanged from a first cell served by a first radio station to the thirdcell while a radio terminal is using the first cell as the primary celland using a second cell served by a second radio station as a secondarycell, perform at least one of (a) receiving communication statusinformation regarding the radio terminal on the second cell and (b)sending to the second radio station an instruction to resume datacommunication which has been provided to the radio terminal in thesecond cell.

In a fifth aspect, a radio terminal includes radio communication unitconfigured to communicate with first to third radio stations, and acommunication control unit. The communication control unit is configuredto perform control of, while using a first cell of the first radiostation as a primary cell, using a second cell served by the secondradio station as a secondary cell. The communication control unit isfurther configured to, when the first cell is being used as the primarycell and the second cell is being used as the secondary cell, change theprimary cell from the first cell to a third cell served by the thirdradio station while keeping communication status information regardingthe radio terminal on the second cell.

In a sixth aspect, a communication control method, in a first radiostation that serves a first cell, includes when changing a primary cellfrom the first cell to a third cell served by a third radio stationwhile a radio terminal is using the first cell as the primary cell andthe a second cell served by a second radio station as a secondary cell,sending to the second radio station at least one of (a) an instructionto abort or suspend communication with the radio terminal in the secondcell and (b) a request to report communication status informationregarding the radio terminal on the second cell.

In a seventh aspect, a communication control method, in a second radiostation that serves a second cell, includes: when a primary cell ischanged from a first cell served by a first radio station to a thirdcell served by a third radio station while a radio terminal is using thefirst cell as the primary cell and using the second cell as a secondarycell, receiving from the first radio station an instruction to abort orsuspend data communication with the radio terminal in the second cell;and aborting or suspending the data communication.

In an eighth aspect, a communication control method, in a third radiostation that serves a third cell, includes when a primary cell ischanged from a first cell served by a first radio station to the thirdcell while a radio terminal is using the first cell as the primary celland using a second cell served by a second radio station as a secondarycell, performing at least one of (a) receiving communication statusinformation regarding the radio terminal on the second cell and (b)sending to the second radio station an instruction to resume datacommunication which has been provided in the second cell.

In a ninth aspect, a communication control method, in a radio terminal,includes: (a) while using a first cell of a first radio station as aprimary cell, using a second cell served by a second radio station as asecondary cell; and (b) when the first cell is being used as the primarycell and the second cell is being used as the secondary cell, changingthe primary cell from the first cell to a third cell served by a thirdradio station while keeping communication status information regardingthe radio terminal on the second cell.

In a tenth aspect, a program includes instructions for causing acomputer to perform the communication control method according to thesixth aspect described above.

In an eleventh aspect, a program includes instructions for causing acomputer to perform the communication control method according to theseventh aspect described above.

In a twelfth aspect, a program includes instructions for causing acomputer to perform the communication control method according to theeighth aspect described above.

In a thirteenth aspect, a program includes instructions for causing acomputer to perform the communication control method according to theninth aspect described above.

Advantageous Effects of Invention

According to the aspects, it is possible to provide a radiocommunication system, a radio station, a radio terminal, a communicationcontrol method, and a program which contribute to enabling acommunication service in a secondary cell to be continued even after aprimary cell is changed when the primary cell is changed duringexecution of carrier aggregation (e.g., Inter-eNB CA) on a plurality ofcells served by different radio stations.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration example of a radiocommunication system according to a first embodiment.

FIG. 2 is a diagram illustrating a configuration example of a firstradio station according to the first embodiment.

FIG. 3 is a diagram illustrating a configuration example of a secondradio station according to the first embodiment.

FIG. 4 is a diagram illustrating a configuration example of a thirdradio station according to the first embodiment.

FIG. 5 is a diagram illustrating a configuration example of a radioterminal according to the first embodiment.

FIG. 6 is a flowchart illustrating an operation example of a radioterminal according to the first embodiment (Procedure Example 1).

FIG. 7 is a flowchart illustrating an operation example of a radionetwork according to the first embodiment (Procedure Example 1).

FIG. 8 is a sequence diagram illustrating an example of a communicationcontrol method in the radio communication system according to the firstembodiment (Procedure Example 1).

FIG. 9 is a sequence diagram illustrating another example of thecommunication control method in the radio communication system accordingto the first embodiment (Modification of Procedure Example 1).

FIG. 10 is a flowchart illustrating an operation example of the radioterminal according to the first embodiment (Procedure Example 2).

FIG. 11 is a flowchart illustrating an operation example of the firstradio station according to the first embodiment (Procedure Example 2).

FIG. 12 is a flowchart illustrating an operation example of the secondradio station according to the first embodiment (Procedure Example 2).

FIG. 13 is a flowchart illustrating an operation example of the thirdradio station according to the first embodiment (Procedure Example 2).

FIG. 14 is a sequence diagram illustrating an example of thecommunication control method in the radio communication system accordingto the first embodiment (Procedure Example 2).

FIG. 15 is a flowchart illustrating an operation example of the radioterminal according to the first embodiment (Procedure Example 2).

FIG. 16 is a flowchart illustrating an operation example of the firstradio station according to the first embodiment (Procedure Example 2).

FIG. 17 is a flowchart illustrating an operation example of the secondradio station according to the first embodiment (Procedure Example 2).

FIG. 18 is a flowchart illustrating an operation example of the thirdradio station according to the first embodiment (Procedure Example 2).

FIG. 19 is a sequence diagram illustrating an example of thecommunication control method in the radio communication system accordingto the first embodiment (Procedure Example 3).

FIG. 20 is a sequence diagram illustrating an example of a communicationcontrol method in a radio communication system according to a secondembodiment (Procedure Example 4).

FIG. 21 is a sequence diagram illustrating an example of thecommunication control method in the radio communication system accordingto the second embodiment (Procedure Example 5).

FIG. 22 is a sequence diagram illustrating an example of thecommunication control method in the radio communication system accordingto the second embodiment (Procedure Example 6).

FIG. 23 is a sequence diagram illustrating an example of thecommunication control method in the radio communication system accordingto the second embodiment (Procedure Example 7).

FIG. 24 is a sequence diagram illustrating a handover procedure incarrier aggregation of LTE (Background Art).

DESCRIPTION OF EMBODIMENTS

Hereinafter, specific embodiments will be described in detail withreference to the drawings. In the respective drawings, the same orcorresponding elements are denoted by the same reference numerals, andredundant description thereof will not be provided as necessary in orderto clarify the description.

First Embodiment

FIG. 1 illustrates a configuration example of a radio communicationsystem according to this embodiment. The radio communication systemaccording to this embodiment includes a radio network 6 and a radioterminal 4. The radio network 6 includes a first radio station 1, asecond radio station 2, a third radio station 3, and a core network 5connected to the radio stations 1 to 3. The radio stations 1, 2, and 3serve a first cell 10, a second cell 20, and a third cell 30,respectively. The radio stations 1 to 3 are, for example, radio basestations or base station controllers. The radio terminal 4 has acapability of using a cell of another radio station as a secondary cell(SCell) while using a cell of one radio station as a primary cell(PCell). In other words, the radio terminal 4 supports carrieraggregation on a plurality of cells which is served by different radiostations. The primary cell (PCell) is a cell which has already been usedas a serving cell of the radio terminal 4 at the start of carrieraggregation. The secondary cell (SCell) is a cell which is additionallyor subordinately used for the carrier aggregation in the radio terminal4.

For example, the radio terminal 4 can establish a second radioconnection in the second cell 20 while maintaining a first radioconnection in the first cell 10 and can use the first cell 10 as theprimary cell (PCell) and the second cell 20 as the secondary cell(SCell). In this way, the radio terminal 4 can use a plurality of cells(e.g., the cells 10 and 20) simultaneously in order to transmit orreceive signals (e.g., user data or control information). The expression“use a plurality of cells simultaneously” is not limited to a case inwhich signals are actually received or transmitted simultaneously in theplurality of cells. The expression may refer to a case in which signalsare received or transmitted in only one or several cells actuallyalthough a state where signals can be received or transmitted in all ofthe plurality of cells is created. The expression may also refer to acase in which different kinds of signals are received or transmitted inrespective cells. Alternatively, the expression may refer to a case inwhich each of the plurality of cells is used for either receiving ortransmitting signals. From the perspective of carrier aggregation on aplurality of cells served by different radio stations, the capability ofusing a plurality of cells served by different radio stations can bereferred to as inter-radio station carrier aggregation. Moreover, fromthe perspective of simultaneous use of a plurality of cells, thecapability of using a plurality of cells served by different radiostations can be referred to as Dual Connection, Dual Connectivity,Multi-Connection, Multi-Connectivity, or the like.

The radio terminal 4 may transmit to a radio station a terminalcapability report indicating that inter-radio station carrieraggregation is supported, or may implicitly indicate that inter-radiostation carrier aggregation is supported by the category, the devicerelease number, or the like of the radio terminal 4. Moreover, asdescribed above, the capability of inter-radio station carrieraggregation can be referred to as dual connection capability ormulti-connection capability.

FIG. 1 illustrates a heterogeneous network (HetNet) environment.Specifically, the first cells 10 and 30 illustrated in FIG. 1 have awider coverage than the second cell 20. Moreover, FIG. 1 illustrates ahierarchical cell structure in which the second cell 20 is deployed nearthe boundary between the first cell 10 and the third cell. However, thecell structure illustrated in FIG. 1 is an example only. In someimplementations, the cells 10, 20, and 30 may have the same degree ofcoverage. In other words, the radio communication system according tothis embodiment may be applied to a homogeneous network environment.

The radio communication system according to this embodiment operates inthe following manner in order to enable a service provided in the SCell(i.e., the cell 20) to be continued even after the PCell is changed whenthe radio terminal 4 changes the PCell from the cell 10 to the cell 30during execution of inter-radio station carrier aggregation on the cells10 and 20. That is, when the radio terminal 4 is using the first cell 10as the PCell and the second cell 20 as the SCell, the radio terminal 4and the radio network 6 execute a procedure (e.g., a handover procedure)for changing the PCell from the first cell 10 to the third cell 30 whilekeeping (storing) communication status information regarding the radioterminal 4 on the second cell 20. The radio terminal 4 and the radionetwork 6 may keep the communication status information on the cell 20without releasing it during execution of the PCell change procedure.

More specifically, when performing the inter-radio station carrieraggregation using the cell 10 as a PCell and the cell 20 as a SCell, theradio terminal 4 changes the PCell from the first cell 10 to the thirdcell 30 while keeping the communication status information on the firstcell 10 (i.e., the PCell) and the second cell 20 (i.e., the SCell).Moreover, after changing the PCell to the third cell 30, the radioterminal 4 resumes communication based on the communication statusinformation kept therein. That is, the radio terminal 4 resumes, in thesecond cell 20 or the third cell 30, the continuity of datacommunication (also referred to as data communication service,communication service, or simply service) which has been provided in thesecond cell 20 as the SCell. Here, the continuity of data communicationmay mean a strict sense of continuity, or may mean the continuity ofservices performed by the data communication. Moreover, if transmissionof a certain packet through the cell 20 has failed, the continuity ofdata communication may mean re-transmission of the packet through thecell 20 or 30.

When performing the inter-radio station carrier aggregation with theradio terminal 4, which uses the cell 10 as the PCell and the cell 20 asthe SCell, the radio network 6 changes the PCell of the radio terminal 4from the first cell 10 to the third cell 30 while keeping thecommunication status information regarding the radio terminal 4 on thefirst cell 10 and the second cell 20. Moreover, after changing the PCellof the radio terminal 4 to the third cell 30, the radio network 6resumes the communication based on the communication status informationkept therein. That is, the radio network 6 resumes, in the second cell20 or the third cell 30, the continuity of data communication (alsoreferred to as data communication service, communication service, orsimply service) which has been provided in the second cell 20 as theSCell.

Accordingly, the radio communication system can allow the continuity ofthe data communication performed in the cells 10 and 20 even after thePCell is changed to the cell 30 of the radio station 3 when the radioterminal 4 is performing data communication (also referred to as datacommunication service, communication service, or simply service) usingthe cells 10 and 20 of the radio stations 1 and 2 as the PCell and theSCell, respectively.

In the above description, the third radio station 3 may be the same asthe first radio station 1. In other words, the first cell 10 and thethird cell 30 may be different cells or sectors provided by one radiostation 1.

The communication status information on the SCell (cell 20) kept in theradio terminal 4 and the radio network 6 may be information necessaryfor resuming the data communication, performed in the SCell (cell 20)before the change of the PCell, after the PCell is changed. Thus, thecommunication status information on the SCell may contain, for example,contents similar to communication status information on a source cellwhich is sent from a source cell radio station to a target cell radiostation during a normal handover. The communication status informationon the SCell may be referred to as information indicating acommunication status or service status of the radio terminal 4 on theSCell. The communication status information on the SCell may be acommunication status of each service or may be communication statuses ofa plurality of services. The communication status information mayinclude, for example, at least one of the following informationelements:

-   -   Transmission or reception status of user data (User Plane        (U-plane));    -   Service information;    -   Bearer information; and    -   Radio resource configuration information.

Here, the bearer is, for example, a signaling radio bearer (SRB), a dataradio bearer (DRB), or a network bearer (S1 bearer, E-RAB, or EPSbearer).

In this embodiment, the change procedure for changing the PCell from thecell 10 to the cell 30 may be a general handover procedure.

The radio terminal 4 may keep the communication status information onthe SCell at the time of changing the PCell in the following manner. Theradio terminal 4 may release the bearer (SCell bearer or secondarybearer) in the second cell 20 configured for the radio terminal 4 andkeep the communication status information. Alternatively, the radioterminal 4 may release the bearer (SCell bearer) in the second cell 20but may keep the configuration information of the bearer in the secondcell and the communication status information. Alternatively, the radioterminal 4 may keep the communication status information whilemaintaining the bearer (SCell bearer) in the second cell 20 configuredfor the radio terminal 4. When the SCell bearer is released, the radioterminal 4 may consider and treat the information regarding the SCellbearer as if it were information regarding the bearer (PCell bearer orprimary bearer) in the first cell 10 configured for the radio terminal4. For example, the radio terminal 4 may treat bearer configurationinformation, excluding radio bearer configuration, similarly to that ofthe PCell bearer. In other words, processes such as keeping, updating,resetting, or reconfiguring of SCell bearer configuration informationmay be performed similarly to those of PCell bearer configurationinformation. The PCell bearer (or the SCell bearer) may be, but is notlimited to, a radio bearer or a network bearer configured in the PCell(or the SCell), for example.

The radio network 6 may keep the communication status information on theSCell by the same method as the radio terminal 4. That is, the radionetwork 6 may release the bearer (SCell bearer) in the second cell 20configured for the radio terminal 4 and may keep the communicationstatus information. Alternatively, the radio network 6 may release thebearer (SCell bearer) in the second cell 20 but may keep theconfiguration information regarding the bearer in the second cell andthe communication status information. Alternatively, the radio network 6may keep the communication status information while maintaining thebearer (SCell bearer) in the second cell 20 configured for the radioterminal 4. When the SCell bearer is released, the radio network 6 mayreconfigure the released SCell bearer as the bearer (PCell bearer) inthe first cell 10 to thereby treat the released SCell bearer as thePCell bearer. For example, the radio network 6 may change the SCellbearer to the PCell bearer by switching the path (route) of the SCellbearer to the PCell and may inherit the configuration information on theSCell bearer, excluding the radio bearer configuration, to the PCellbearer. That is, the bearer configuration information regarding theSCell bearer, excluding the radio bearer configuration, is inherited tothe PCell bearer. The PCell bearer (or the SCell bearer) may be, but isnot limited to, a radio bearer or a network bearer configured in thePCell (or the SCell), for example.

As described above, the data communication which has been provided inthe second cell 20 as the SCell before the change of the PCell may beresumed in the third cell 30 (i.e., the PCell after the change) afterthe PCell is changed. In this case, the radio network 6 may transfer,from the radio station 2 to the radio station 3, the communicationstatus information regarding the radio terminal 4 on the cell 20.Instead of this, the data communication which has been provided in thesecond cell 20 may be resumed in the second cell 20 after the PCell ischanged. In this case, the radio network 6 may keep, in the radiostation 2, the communication status information regarding the radioterminal 4 on the cell 20. After having changed the PCell, the radionetwork 6 may perform carrier aggregation with the radio terminal 4using the cell 30 as the PCell and the cell 20 as the SCell.

Hereinafter, the data communication will be described as a broad senseof service. From the perspective of a radio network, the servicereferred herein includes, but is not limited to, transmission ofdownlink data and control-plane signaling, reception of uplink data andcontrol-plane signaling, and transmission and reception of downlink anduplink voice calls. Similarly, from the perspective of a radio terminal,the service includes, but is not limited to, reception of downlink dataand control signals, transmission of uplink data and control signals,and reception and transmission of downlink and uplink voice calls.Hereinafter, the embodiments of the present invention are basicallydescribed by way of an example of a downlink service. However, thepresent invention can naturally be applied to an uplink service.

Next, configuration examples of the radio stations 1 to 3 and the radioterminal 4 according to this embodiment will be described. FIG. 2 is ablock diagram illustrating a configuration example of the first radiostation 1. A radio communication unit 11 receives an uplink signaltransmitted from the radio terminal 4 via an antenna. A reception dataprocessing unit 13 restores the received uplink signal. Obtainedreceived data is transferred to another network node (e.g., a datatransfer apparatus or a mobility management apparatus in the corenetwork 5) or another radio station via a communication unit 14. Forexample, uplink user data received from the radio terminal 4 istransferred to a data transfer apparatus in the core network 5.Moreover, non-access stratum (NAS) control data among control datareceived from the radio terminal 4 is transferred to a mobilitymanagement apparatus in the core network 5. Further, the reception dataprocessing unit 13 receives, from a communication control unit 15,control data to be transmitted to the radio station 2 or 3 and sends thecontrol data to the radio station 2 or 3 via the communication unit 14.

A transmission data processing unit 12 acquires user data destined forthe radio terminal 4 from the communication unit 14 and performsprocesses such as error correction coding, rate matching, andinterleaving to generate a transport channel. Further, the transmissiondata processing unit 12 then generates a transmission symbol sequence byadding control information to the data sequence of the transportchannel. The radio communication unit 11 generates a downlink signal byperforming processing such as carrier wave modulation based on thetransmission symbol sequence, frequency conversion, and signalamplification, and transmits the generated downlink signal to the radioterminal 4. Furthermore, the transmission data processing unit 12receives control data to be transmitted to the radio terminal 4 from thecommunication control unit 15 and transmits the control data to theradio terminal 4 via the radio communication unit 11.

The communication control unit 15 controls the inter-radio stationcarrier aggregation which uses the first cell 10 as the PCell and thesecond cell 20 as the SCell. Further, the communication control unit 15performs a procedure for changing the PCell of the radio terminal 4 fromthe first cell 10 to the third cell 30 while keeping the communicationstatus information regarding the radio terminal 4 on the SCell (secondcell 20) in the radio network 6 and the radio terminal 4. Thecommunication control unit 15 performs the PCell change procedure incooperation with the radio station 2, the radio station 3, and the radioterminal 4.

In an example, the communication control unit 15 may receive thecommunication status information regarding the radio terminal 4 on theSCell (second cell 20) from the second radio station 2, and then sendthe communication status information to the third radio station 3.Moreover, the communication control unit 15 may request the core network5 or the second radio station 2 to switch the bearer in the cell 20(SCell bearer) to a bearer in the cell 10 (PCell bearer). In anotherexample, the communication control unit 15 may instruct the second radiostation 2 to suspend providing a service in the SCell while keeping thecommunication status information regarding the radio terminal 4 on theSCell (second cell 20). The details of control and signaling performedby the communication control unit 15 will be described later.

FIG. 3 is a block diagram illustrating a configuration example of thesecond radio station 2. The functions and operations of a radiocommunication unit 21, a transmission data processing unit 22, areception data processing unit 23, and a communication unit 24illustrated in FIG. 3 are similar to those of the corresponding elementsof the radio station 1 illustrated in FIG. 2, i.e., the radiocommunication unit 11, the transmission data processing unit 12, thereception data processing unit 13, and the communication unit 14.

A communication control unit 25 of the radio station 2 controls theinter-radio station carrier aggregation which uses the first cell 10 asthe PCell and the second cell 20 as the SCell. Further, thecommunication control unit 25 performs a procedure for changing thePCell of the radio terminal 4 from the first cell 10 to the third cell30 while keeping the communication status information regarding theradio terminal 4 on the SCell (second cell 20) in the radio network 6and the radio terminal 4. The communication control unit 25 performs thePCell change procedure in cooperation with the radio station 1, theradio station 3, and the radio terminal 4.

In an example, the communication control unit 25 may operate so as tosend the communication status information regarding the radio terminal 4on the SCell (second cell 20) to the first radio station 1 or the corenetwork 5. Moreover, the communication control unit 25 may request thecore network 5 or the first radio station 1 to switch the bearer in thecell 20 (SCell bearer) to the bearer in the cell 10 (PCell bearer).Further, the communication control unit 25 may receive the communicationstatus information regarding the radio terminal 4 on the SCell from thecore network 5 or the third radio station 3 after the PCell is changed.In another example, the communication control unit 25 may suspend theprovision of a service to the radio terminal 4 in the SCell (second cell20), keep the communication status information regarding the radioterminal 4 on the SCell (second cell 20), and resume provision of theservice suspended in the second cell 20 after the PCell is changed fromthe first cell 10 to the third cell 30. The details of control andsignaling performed by the communication control unit 25 will bedescribed later.

FIG. 4 is a block diagram illustrating a configuration example of thethird radio station 3. The functions and operations of a radiocommunication unit 31, a transmission data processing unit 32, areception data processing unit 33, and a communication unit 34illustrated in FIG. 4 are similar to those of the corresponding elementsof the radio station 1 illustrated in FIG. 2, i.e., the radiocommunication unit 11, the transmission data processing unit 12, thereception data processing unit 13, and the communication unit 14.

The communication control unit 35 of the radio station 3 performs aprocedure for changing the PCell of the radio terminal 4 from the firstcell 10 to the third cell 30 while keeping the communication statusinformation regarding the radio terminal 4 on the SCell (second cell 20)in the radio network 6 and the radio terminal 4. The communicationcontrol unit 35 performs the PCell change procedure in cooperation withthe radio station 1, the radio station 2, and the radio terminal 4.

In an example, the communication control unit 35 may receive thecommunication status information regarding the radio terminal 4 on theSCell (second cell 20) from the first radio station 1. The communicationcontrol unit 35 may perform control so as to allow the service, whichhas been provided in the SCell (second cell 20) before the change of thePCell, to be provided in the PCell after the change (i.e., the thirdcell 30) based on the communication status information on the SCell.Alternatively, the communication control unit 35 may perform control soas to allow the service, which has been provided in the SCell (secondcell 2) before the change of the PCell, to be provided in the secondcell again. In this case, the communication control unit 35 may controlthe inter-radio station carrier aggregation which uses the third cell 30as the PCell and uses the second cell 20 as the SCell. The details ofcontrol and signaling performed by the communication control unit 35will be described later.

FIG. 5 is a block diagram illustrating a configuration example of theradio terminal 4. The radio communication unit 41 supports carrieraggregation on a plurality of cells served by different radio stationsand can use the plurality of cells (e.g., the cells 10 and 20)simultaneously in order to transmit or receive user data. Specifically,the radio communication unit 41 receives a downlink signal from theradio station 1, the radio station 2, or the radio station 3 via anantenna. The reception data processing unit 42 restores received datafrom the received downlink signal, and sends the received data to a datacontrol unit 43. The data control unit 43 uses the received dataaccording to the purpose thereof. A transmission data processing unit 44and the radio communication unit 41 generate an uplink signal usingtransmission data supplied from the data control unit 43, and transmitthe uplink signal to the radio station 1, the radio station 2, or theradio station 3.

The communication control unit 45 of the radio terminal 4 controlscarrier aggregation using a plurality of cells as a PCell and a SCell.For example, the communication control unit 45 controls the inter-radiostation carrier aggregation which uses the first cell 10 as the PCelland the second cell 20 as the SCell. Further, the communication controlunit 45 performs a procedure for changing the PCell of the radioterminal 4 from the first cell 10 to the third cell 30 while keeping thecommunication status information regarding the radio terminal 4 on theSCell (second cell 20) in the radio network 6 and the radio terminal 4.The communication control unit 45 performs the PCell change procedure incooperation with the radio station 1, the radio station 2, and the radiostation 3.

In an example, the communication control unit 45 may change the PCellfrom the first cell 10 to the third cell 30 while keeping thecommunication status information regarding the radio terminal 4 on theSCell (second cell 20). Specifically, the communication control unit 45may keep the communication status information regarding the radioterminal 4 on the SCell (second cell 20) without releasing it during theexecution of the PCell change procedure (e.g., a handover procedure).After having changed the PCell to the third cell 30, the communicationcontrol unit 45 may resume, in the second cell 20 or the third cell 30,the service which has been provided in the SCell (second cell 20) beforethe change of the PCell. The details of control and signaling performedby the communication control unit 45 will be described later.

Hereinafter, Procedure Examples 1 to 3 of a communication control methodin the radio communication system according to this embodiment will bedescribed.

Procedure Example 1

In Procedure Example 1, after a handover of the radio terminal 4 fromthe first cell 10 to the third cell 30, the radio network 6 resumes, inthe third cell 30, the communication (service) which has been providedin the second cell 20 as the SCell before the handover.

FIG. 6 is a flowchart illustrating an example of an operation of theradio terminal 4 according to Procedure Example 1. In step S101, theradio terminal 4 (communication control unit 45) receives from the radionetwork 6 an instruction to perform a handover to the third cell 30. Instep S102, the radio terminal 4 (communication control unit 45)initiates a handover to the cell 30 while keeping the communicationstatus information on the cells 10 and 20. When the handover iscompleted (YES in step S103), the radio terminal 4 (communicationcontrol unit 45) resumes the communication of the radio terminal 4 inthe cell 30 based on the kept communication status information on thecells 10 and 20. The resumed communication includes a communicationwhich has been performed in the SCell (cell 20) before the change of thePCell.

FIG. 7 is a flowchart illustrating an example of an operation of theradio network 6 according to Procedure Example 1. In step S201, theradio network 6 (e.g., the communication control unit 15 of the radiostation 1) transmits, to the radio terminal 4, an instruction to performa handover to the third cell 30. In step S202, while keeping thecommunication control information regarding the radio terminal 4 on thecells 10 and 20, the radio network 6 (e.g., the communication controlunit 15 of the radio station 1, the communication control unit 25 of theradio station 2, and the communication control unit 35 of the radiostation 3) initiates a handover procedure of the radio terminal 4 fromthe cell 10 to the cell 30. When the handover is completed (YES in stepS203), the radio network 6 (communication control unit 35) performscommunication with the radio terminal 4 in the cell 30 based on the keptcommunication status information regarding the radio terminal 4 on thecells 10 and 20. That is, the radio network 6 resumes the communication.The resumed communication includes a communication which has beenperformed in the SCell (cell 20) before the change of the PCell.

FIG. 8 is an example of a sequence diagram illustrating the entireprocess of Procedure Example 1. In steps S301 and S302, the radionetwork 6 and the radio terminal 4 performs communication (inter-radiostation carrier aggregation) using the first cell 10 as the PCell andthe second cell 20 as the SCell. In step S303, the radio network 6transmits, to the radio terminal 4, an instruction to perform a handoverfrom the cell 10 to the cell 30. In step S304, the radio terminal 4initiates the handover to the cell 30 while keeping the communicationstatus information on the cells 10 and 20. The radio network 6 alsokeeps the communication status information regarding the radio terminal4 on the cells 10 and 20 during the handover of the radio terminal 4(step S305). In step S306, the radio network 6 and the radio terminal 4complete the handover of the radio terminal 4 from the cell 10 to thecell 30. In step S307, the radio network 6 and the radio terminal 4resume the communication of the radio terminal 4 in the cell 30 based onthe kept communication status information regarding the radio terminal 4on the cells 10 and 20. The resumed communication includes acommunication which has been performed in the SCell (cell 20) before thechange of the PCell.

In Procedure Example 1 described above, the first radio station 1(communication control unit 15) may explicitly inform the radio terminal4 to keep the communication status information on the second cell 20 (orto perform a handover while keeping the communication statusinformation). This notification may be transmitted together with amessage of an instruction to perform a handover from the first cell 10to the third cell 30 or may be transmitted using a message differentfrom the handover instruction. Moreover, the first radio station 1(communication control unit 15) may inform the radio terminal 4 torelease a bearer (SCell bearer) in the cell 20 or to maintain the SCellbearer.

Modification of Procedure Example 1

In a modification of Procedure Example 1, after a handover of the radioterminal 4 from the first cell 10 to the third cell 30, the radionetwork 6 resumes, in the second cell 20, the communication (service)which has been provided in the second cell 20 as the SCell before thehandover. That is, the third radio station 3 uses the second cell 20 asa SCell after the handover to change the PCell of the radio terminal 4to the third cell 30 is performed.

FIG. 9 is a sequence diagram illustrating the entire process of themodification of the Procedure Example 1. The processes of steps S401 toS407 of FIG. 9 are similar to the processes of steps S301 to S307illustrated in FIG. 8. In step S408, after changing the PCell of theradio terminal 4, the radio network 6 (e.g., the communication controlunit 35 of the radio station 3) transmits, to the radio terminal 4, aninstruction to start using the second cell 20. Instead of this, theradio network 6 (e.g., the communication control unit 35 of the radiostation 3) may transmit, to the radio terminal 4, an instruction toresume the communication in the cell 20. In this way, in step S409, theradio network 6 and the radio terminal 4 can resume, in the cell 30, thecommunication which has been performed in the cell 10 before the changeof the PCell, and resume, in the cell 20, the communication which hasbeen performed in the cell 20 before the change of the PCell.

After the change (i.e., the handover) of the PCell of the radio terminal4, the radio network 6 may transmit, to the radio terminal 4, radioresource configuration information regarding the second cell 20 in orderto start using the second cell 20. Moreover, the transmission of theradio resource configuration information regarding the second cell 20may indicate the instruction to start using the second cell from theradio network 6 to the radio terminal 4. Instead of this, the radionetwork 6 may transmit to the radio terminal 4 an activation messageindicating activation of the second cell 20 separately from the radioresource configuration information regarding the second cell 20.

Procedure Example 2

Procedure Example 2 corresponds to a more specific example of ProcedureExample 1 described above. FIG. 10 is a flowchart illustrating anexample of the operation of the radio terminal 4 according to ProcedureExample 2. In step S501, the radio terminal 4 (communication controlunit 45) receives, on the first cell 10 from the first radio station 1,an instruction to perform a handover to the third cell 30. In step S502,the radio terminal 4 (communication control unit 45) keeps thecommunication status information regarding the communication (service A)provided in the PCell (first cell 10) and the communication (service B)provided in the SCell (second cell 20). In step S503, the radio terminal4 (communication control unit 45) initiates a handover from the firstcell 10 to the third cell 30. When the handover is completed (YES instep S504), the radio terminal 4 (communication control unit 45) resumesthe services A and B in the cell 30 based on the kept communicationstatus information on the cells 10 and 20.

FIG. 11 is a flowchart illustrating an example of the operation of thefirst radio station 1 according to Procedure Example 2. In step S601,the radio station 1 (communication control unit 15) sends, to the radiostation 3, a handover request indicating a handover of the radioterminal 4 to the third cell 30. In step S602, the radio station 1(communication control unit 15) receives from the radio station 3 apositive response (ACK) to the handover request. In step S603, the radiostation 1 (communication control unit 15) sends, to the radio station 2,an instruction to abort the service for the radio terminal 4 and aninstruction to report the communication status information regarding theradio terminal 4. In step S604, the radio station 1 (communicationcontrol unit 15) receives, from the radio station 2, the report of thecommunication status information regarding the radio terminal 4 on thesecond cell 20. In step S605, the radio station 1 (communication controlunit 15) transmits, to the radio terminal 4, an instruction to perform ahandover to the third cell 30. In step S606, the radio station 1(communication control unit 15) sends, to the radio station 3, thecommunication status information regarding the radio terminal 4 on thefirst cell 10 managed by itself and the communication status informationregarding the radio terminal 4 on the second cell 20 received from theradio station 2. In step S607, the radio station 1 (communicationcontrol unit 15) ends the process of FIG. 11 in response to receiving ahandover completion notification from the radio station 3.

FIG. 12 is a flowchart illustrating an example of the operation of thesecond radio station 2 according to Procedure Example 2. In step S701,the radio station 2 (communication control unit 25) receives, from theradio station 1, the instruction to abort the service for a target radioterminal (i.e., the radio terminal 4) and the instruction to report thecommunication status information regarding the radio terminal 4. In stepS702, the radio station 2 (communication control unit 25) aborts theprovision of the service to the radio terminal 4 in the second cell 20.In step S703, the radio station 2 (communication control unit 25) sends,to the radio station 1, the communication status information regardingthe radio terminal 4 on the second cell 20.

FIG. 13 is a flowchart illustrating an example of the operation of thethird radio station 3 according to Procedure Example 2. In step S801,the radio station 3 (communication control unit 35) receives, from theradio station 1, a handover request indicating a handover of the radioterminal 4 to the third cell 30. In step S802, the radio station 3(communication control unit 35) sends, to the radio station 1, apositive response (ACK) to the handover request. In step S803, the radiostation 3 (communication control unit 35) receives, from the radiostation 1, the communication status information regarding the radioterminal 4 on the cells 10 and 20. In step S804, the radio station 3(communication control unit 35) performs a handover procedure of theradio terminal 4. When the handover is completed (Yes in step S804), theradio station 3 (communication control unit 35) sends a handovercompletion notification to the radio station 1 (step S805). In stepS806, the radio station 3 (communication control unit 35) resumes theservices A and B in the third cell 30 based on the communication statusinformation regarding the radio terminal 4 on the cells 10 and 20.

FIG. 14 is an example of a sequence diagram illustrating the entireprocess of Procedure Example 2. In step S901, the radio station 1 andthe radio terminal 4 performs communication (service A) using the firstcell 10 as the PCell. In step S902, the radio station 2 and the radioterminal 4 performs communication (service B) using the second cell 20as the SCell. In step S903, the radio station 1 sends a handover requestto the radio station 3, and receives a response (ACK) to the handoverrequest from the radio station 3. In step S904, the radio station 1sends, to the radio station 2, an instruction to abort the service B andan instruction to report the communication status information. In stepS905, in response to the service abort instruction, the radio station 2aborts the service B for the radio terminal 4 in the SCell (second cell20). In step S906, in response to the instruction to report thecommunication status information, the radio station 2 reports to theradio station 1 the communication status information regarding the radioterminal 4 on the second cell 20.

In step S907, the radio station 1 aborts the service A in the PCell(first cell 10). In step S908, the radio station 1 transmits a handoverinstruction to the radio terminal 4. In step S909, the radio terminal 4performs a handover from the cell 10 to the cell 30 while keeping thecommunication status information on the cells 10 and 20. In step S910,the radio station 1 sends, to the radio station 3, the communicationstatus information regarding the radio terminal 4 on the cells 10 and20. In step S911, the radio station 3 and the radio terminal 4 completethe handover. In step S912, the radio station 3 sends a handovercompletion notification to the radio station 1. In step S913, the radiostation 3 and the radio terminal 4 resumes, in the cell 30, thecommunication (services A and B) which has been performed in the cells10 and 20 before the handover.

In Procedure Example 2 described above, the radio station 1 may sendsonly the instruction to abort the service B to the radio station 2instead of sending both the instruction to abort the service B and theinstruction to report the communication status of the radio terminal 4.In this case, the radio station 1 sends only the communication statusinformation on the first cell to the radio station 3. Further, in thiscase, the radio station 3 may send, to the radio station 2, aninstruction to report the communication status of the radio terminal 4.

For example, the radio station 3 may send this instruction afterreceiving the communication status on the first cell from the radiostation 1, or before or after sending the handover completionnotification to the radio station 1.

Procedure Example 3

In Procedure Example 3, the second radio station 2 temporarily suspendsthe communication (service B) in the second cell 20 and resumes thecommunication (service B) which has been suspended in the second cell 20after the PCell of the radio terminal 4 is changed from the first cell10 to the third cell 30. Specifically, the third radio station 3transmits, to the radio terminal 4, an instruction to resumecommunication (to resume service) in the second cell 20 after completionof the handover of the radio terminal 4. The third radio station 3 maytransmit either one or both of radio resource configuration informationand radio resource control information on the second cell 20 togetherwith or instead of an instruction to start using the second cell 20 oran instruction to resume communication (to resume service).

FIG. 15 is a flowchart illustrating an example of the operation of theradio terminal 4 according to Procedure Example 3. The processes ofsteps S1001 to S1002 illustrated in FIG. 15 are similar to the processesof steps S501 to S502 illustrated in FIG. 10. In step S1003, the radioterminal 4 (communication control unit 45) keeps configurationinformation (e.g., radio resource configuration information) of thesecond cell 20. The processes of steps S1004 to S1005 are similar to theprocesses of steps S503 to S504 illustrated in FIG. 10. In step S1006,the radio terminal 4 (communication control unit 45) resumes the serviceA in the third cell 30 based on the communication status information onthe first cell 10. In step S1007, the radio terminal 4 (communicationcontrol unit 45) determines whether an instruction to resumecommunication in the second cell 20 has been received from the radiostation 3. When the instruction to resume communication has beenreceived (YES in step S1007), the radio terminal 4 (communicationcontrol unit 45) resumes the service B in the second cell 20 based onthe communication status information and the configuration informationon the second cell 20 (step S1008).

FIG. 16 is a flowchart illustrating an example of the operation of theradio station 1 according to Procedure Example 3. The processes of stepsS1101, S1102, S1105, and S1107 illustrated in FIG. 16 are similar to theprocesses of steps S601, S602, S605, and S607 illustrated in FIG. 11. Instep S1103, after receiving a positive response (ACK) to the handoverrequest, the radio station 1 (communication control unit 15) sends, tothe radio station 2, an instruction to suspend the service B. In stepS1104, the radio station 1 (communication control unit 15) receives thereport of the service suspension from the radio station 2. In stepS1106, the radio station 1 sends to the radio station 3 thecommunication status information on the PCell (first cell 10) managed byitself. Since the communication status information on the SCell (secondcell 20) is kept in the radio station 2, the radio station 1 may notsend the communication status information on the SCell to the radiostation 3.

FIG. 17 is a flowchart illustrating an example of the operation of theradio station 2 according to Procedure Example 3. In step S1201, theradio station 2 (communication control unit 25) receives from the radiostation 1 an instruction to suspend the service B. In step S1202, theradio station 2 (communication control unit 25) suspends the service Bfor the target radio terminal (i.e., the radio terminal 4) in the secondcell 20 in response to the suspension instruction. In step S1203, theradio station 2 (communication control unit 25) sends the report of thesuspension of the service B in the second cell 20 to the radio station1. In step S1204, the radio station 2 (communication control unit 25)determines whether an instruction to resume the service B has beenreceived from the radio station 3. When the instruction to resume hasbeen received (YES in step S1204), the radio station 2 (communicationcontrol unit 25) resumes the service B in the second cell 20 based onthe communication status information regarding the radio terminal 4 onthe cell 20 managed by itself.

FIG. 18 is a flowchart illustrating an example of the operation of theradio station 3 according to Procedure Example 3. The processes of stepsS1301 to S1305 illustrated in FIG. 18 are similar to the processes ofsteps S801 to S805 illustrated in FIG. 13. In step S1306, the radiostation 3 (communication control unit 35) resumes the service A in thethird cell 30 based on the communication status information regardingthe radio terminal 4 on the cell 10. In step S1307, the radio station 3(communication control unit 35) sends, to the radio station 2, aninstruction to resume the service B for the radio terminal 4 in thesecond cell 20.

FIG. 19 illustrates an example of a sequence diagram illustrating theentire process of Procedure Example 3. The processes of steps S1401 toS1403 illustrated in FIG. 19 are similar to the processes of steps S901to S903 of FIG. 14 according to Procedure Example 2. In step S1404 ofFIG. 19, the radio station 1 sends to the radio station 2 an instructionto suspend the service B instead of an instruction to abort the serviceB. Moreover, it is not necessary to transmit an instruction to reportthe communication status information. The radio station 2 suspends theservice B in response to the suspension instruction (step S1405) andsends the report of the service suspension to the radio station 1 (stepS1406). After step S1406, the processes of steps S1407 to S1409 similarto the processes of steps S907 to S909 of FIG. 14 are performed.

In step S1410 of FIG. 19, the radio station 1 sends to the radio station3 the communication status information regarding the radio terminal 4 onthe PCell (first cell 10). It is not necessary to send the communicationstatus information regarding the radio terminal 4 on the SCell (secondcell 20). After step S1410, the processes of steps S1411 and S1412similar to the processes of steps S911 and S912 of FIG. 14 areperformed. In step S1413 of FIG. 19, the radio station 3 and the radioterminal 4 resume the service A of the PCell in the third cell 30. Instep S1414, the radio station 3 sends, to the radio station 2, aninstruction to resume the service B of the SCell. In step S1415, theradio station 2 and the radio terminal 4 resume the service B of theSCell in the second cell 20.

The communication (service) performed by the radio terminal 4 inProcedure Examples 1 to 3 described above may be data communication(User plane (U-plane)) or may be control-plane signaling (Control plane(C-plane)).

In Procedure Examples 1 to 3 described above, exchange of messages andinformation between radio stations may be performed through the corenetwork 5.

In Procedure Example 1 described above, the radio network 6 and theradio terminal 4 may further perform communication in a fourth cellserved by a fourth radio station deployed within the area (coverage) ofthe third cell 30.

In Procedure Examples 2 and 3 described above, the service A and theservice B may be the same service.

In Procedure Example 3 described above, both services A and B may beperformed in the cell 20 after the change of the PCell (after thehandover of the radio terminal 4).

Procedure Examples 1 to 3 described above can be applied to, but notlimited to, a case in which the first radio station 1 is a radio stationthat serves (manages) a cell having a relatively large coverage and thesecond radio station 2 is a low-power radio station (Low Power Node(LPN)) that serves (manages) a cell having a small coverage. Examples ofa LPN include a radio station having the same functions as the radiostation 1 and a new type of network node (New Node) having fewerfunctions than the radio station 1. Moreover, the second cell 20 may bea new type cell (New Cell Type) which is different from existing cellsand uses a new type of carrier (New Carrier Type) different from anexisting carrier.

Second Embodiment

In this embodiment, an example in which the first embodiment describedabove is applied to a 3GPP LTE system will be described. A configurationexample of a radio communication system according to this embodiment maybe similar to that illustrated in FIG. 1. However, the radio stations 1to 3 correspond to eNBs, the radio terminal 4 corresponds to a UE, andthe core network 5 corresponds to an evolved packet core (EPC).Moreover, eNB1 to eNB3 correspond to a radio access network (RAN).Transmission and reception of information between radio stations (i.e.,between eNBs) may use an X2 interface which is a direct interface, mayuse an S1 interface through a core network, or may use a newly definedinterface (e.g., an X3 interface). The radio terminal (UE) 4 supportscarrier aggregation (Inter-eNB CA) on a plurality of cells served bydifferent radio stations (eNBs). The expression “Inter-eNB CA” is notlimited to a case in which signals are actually received or transmittedin the cells of different eNBs simultaneously. The expression may referto a case in which signals are received or transmitted in cells ofseveral eNBs actually although a state where signals (e.g., user data orcontrol information) can be received or transmitted in all of the cellsof different eNBs is created. The expression may also refer to a case inwhich different kinds of signals are received or transmitted inrespective cells of different eNBs. Alternatively, the expression mayrefer to a case in which each of the cells of different eNBs is used foreither receiving or transmitting signals. In the following description,the radio stations 1 to 3 are referred to as eNB1 to eNB3, the radioterminal 4 are referred to as a UE 4, and the core network 5 will bereferred to as an EPC 5.

As described in the first embodiment, the communication statusinformation may include, for example, at least one of the followinginformation elements:

-   -   Transmission or reception status of user data (User Plane        (U-plane));    -   Service information;    -   Bearer information; and    -   Radio resource configuration information.

The transmission or reception status of user data may be, for example,contents of a SN Status Transfer message that transfers informationindicating statuses of a packet data convergence protocol (PDCP)sequence number (SN) and a hyper frame number (HFN). The SN StatusTransfer message includes an E-RAB ID, a Receive Status Of UL PDCP SDUs,an UL COUNT Value (PDCP SN+HFN), a DL COUNT Value (PDCP SN+HFN), and thelike. Moreover, the transmission or reception status of the user datamay be an RLC status (e.g., RLC STATUS PDU).

The service information may include QoS information or a QoS ClassIndicator (QCI) value.

The bearer information is information on a signaling radio bearer (SRB),a data radio bearer (DRB), or a network bearer (S1 bearer, E-RAB, or EPSbearer). The bearer information may include, for example, a bearer ID(e.g., drb-Identity, eps-Bearer Identity, E-RAB ID), terminalidentification information (e.g., eNB UE S1AP ID, MME UE S1AP ID, orTMSI), or network identification information (e.g., GUMMEI, UL GTPTunnel Endpoint, or DL GTP Tunnel Endpoint).

The radio resource configuration information may include, for example,common radio resource configuration information (Radio Resource ConfigCommon) or dedicated radio resource configuration information (RadioResource Config Dedicated).

Next, Procedure Examples 4 to 7 of a communication control method in theradio communication system according to this embodiment will bedescribed.

Procedure Example 4

Procedure Example 4 corresponds to Procedure Example 2 described in thefirst embodiment. That is, when the PCell is changed to the cell 30 ofthe eNB3 during the period in which the UE 4 experiences the service Ain the cell 10 (PCell) of the eNB1 and the service B in the cell 20(SCell) of the eNB2, the UE 4 and the radio network 6 (i.e., RAN andEPC) perform a handover of the UE 4 while keeping the communicationstatus information on the cells 10 and 20. The UE 4 and the radionetwork 6 then resume the services A and B in the cell 30 of the eNB3after the PCell is changed by the handover.

FIG. 20 illustrates an example of a sequence diagram illustrating theentire process of Procedure Example 4. In FIG. 20, the first cell 10 isdenoted by CELL1, the second cell 20 is denoted by CELL2, and the thirdcell 30 is denoted by CELL3. In step S1501, the eNB1 and the UE 4perform communication (service A) using the first cell 10 as the PCell.In step S1502, the eNB2 and the UE 4 perform communication (service B)using the second cell 20 as the SCell. In step S1503, the UE 4 transmitsa measurement report to the eNB1. In step S1504, the eNB1 determines ahandover of the UE 4 to the third cell 30 based on the measurementreport and sends a handover request (HO preparation request) to theeNB3. In step S1505, the eNB1 receives from the eNB3 a response (HOpreparation request ACK) to the handover request. In step S1506, theeNB1 sends to the eNB2 an instruction to abort the service B and aninstruction to report the communication status information (ServiceAbort Request and Communication Status Request). In step S1507, the eNB2aborts the service B in the cell 20 and sends to the eNB1 thecommunication status information regarding the UE 4 on the cell 20. Instep S1508, the eNB1 aborts the service A in the cell 10 and sends ahandover instruction (RRC Connection Reconfiguration) to the UE 4. Here,the service abort (Service Abort Request) may be newly defined as aCause value.

In steps S1509 and S1510, the UE 4 releases the bearer in the cell 20(Release Bearer in Cell2) and performs a handover from the cell 10 tothe cell 30 while keeping the communication status information on thecells 10 and 20. In step S1511, the eNB1 sends a SN STATUS TRANSFERmessage to the eNB3. The SN STATUS TRANSFER message includes thecommunication status information regarding the UE 4 on the cells 10 and20. In step S1512, the eNB3 and the UE 4 establish synchronization inthe cell 30 and complete the handover process (Synchronization and RRCConnection Reconfiguration Complete). In step S1513, the eNB3 sends ahandover completion notification (UE Context Release) to the eNB1. Instep S1514, the eNB3 and the UE 4 resume (continue) the services A and Bin the cell 30.

The release of the SCell Bearer by the UE 4 in step S1510 of FIG. 20 maybe performed by releasing the radio resource configuration (e.g., RadioResource Config Common, Radio Resource Config Dedicated), for example.Moreover, the release of the SCell bearer may be performed by releasingeither one or both of the Data Radio Bearer (DRB) and Signaling RadioBearer (SRB) established in the cell 20.

The resuming (continuing) of the service B in step S1514 of FIG. 20 maybe performed by re-establishing the Packet Data Convergence Protocol(PDCP) layer and Radio Link Control (RLC) layer corresponding to thebearer (e.g., a radio Bearer) established in the cell 20. Thus, theresuming of the service B in step S1514 can be also referred to asbearer re-establishment or bearer re-configuration.

In Procedure Example 4 described above, the eNB1 may sends only theinstruction to abort the service B to the eNB2 instead of sending boththe instruction to abort the service B and the instruction to report thecommunication status of the UE 4. In this case, the eNB1 may send onlythe communication status information on the Cell1 to the eNB3. Further,in this case, the eNB3 may send, to the eNB2, the instruction to reportthe communication status of the UE 4. For example, the eNB3 send thisinstruction after receiving the report of the communication status inthe Cell1 from the eNB1, or before or after sending the handovercompletion notification to the eNB1.

Procedure Example 5

In Procedure Example 5, the process of the EPC 5 is added to ProcedureExample 4. FIG. 21 is an example of a sequence diagram illustrating theentire process of Procedure Example 5. In FIG. 21, the cells 10, 20, and30 are denoted by CELL1, CELL2, and CELL3, respectively. The processesof steps S1601 to S1603 of FIG. 21 are similar to the processes of stepsS1501 to S1505 of FIG. 20.

In step S1604 of FIG. 21, the eNB1 sends, to the EPC 5 (e.g., a MobilityManagement Entity (MME)), a bearer switch request for switching thebearer of the radio terminal 4, which passes through the cell 20 of theeNB2, to a bearer through the cell 10 of the eNB1 (Path switch of bearerat eNB2/Cell2 to eNB1/Cell1). Alternatively, the eNB2 may send thebearer switch request to the EPC 5.

The processes of steps S1605 to S1611 of FIG. 21 are similar to theprocesses of steps S1506 to S1512 of FIG. 20. In step S1612 of FIG. 21,the eNB3 sends, to the EPC 5 (e.g., an MME), a bearer switch request forswitching the bearer of the radio terminal 4, which passes through thecell 10 of the eNB1, to a bearer through the cell 30 of the eNB3 (Pathswitch of bearer at eNB1/Cell1 to eNB3/Cell3). The processes of stepsS1613 to S1614 of FIG. 21 are similar to the processes of steps S1513 toS1514 of FIG. 20.

Procedure Example 6

Procedure Example 6 corresponds to Procedure Example 3 described in thefirst embodiment. That is, after the eNB2 temporarily suspends thecommunication (service B) in the cell 20 and the PCell of the UE 4 ischanged from the cell 10 to the cell 30, the communication (service B)which has been suspended in the cell 20 is resumed. After the handoverof the UE 4 is completed, the eNB3 transmits to the UE 4 an instructionto resume the communication (to resume service) in the cell 20.

FIG. 22 illustrates an example of a sequence diagram illustrating theentire process of Procedure Example 6. In FIG. 23, the cells 10, 20, and30 are denoted by CELL1, CELL2, and CELL3. The processes of steps S1701to S1705 of FIG. 22 are similar to the processes of steps S1501 to S1505of FIG. 20. In step S1706 of FIG. 22, the eNB1 sends to the eNB2 aninstruction (Service Suspension Request) to suspend the service Binstead of an instruction to abort the service B. In step S1707, theeNB2 suspends the service B and sends to the eNB1 a response (ServiceSuspension Request ACK) to the suspension instruction. The servicesuspension instruction can be also referred to as a bearer suspensioninstruction (Bearer Suspension Request). Moreover, these instructionsmay be newly defined as Cause values.

The processes of steps S1708 to S1709 of FIG. 22 are similar to theprocesses of steps S1508 to S1509 of FIG. 20. In Procedure Example 6,the UE 4 does not need to release the bearer in the cell 20. That is,the UE 4 may maintain the bearer in the cell 20 in addition to thecommunication status information on the cell 20. However, the UE 4 maytemporarily release the bearer in the cell 20.

In step S1710 of FIG. 22, the eNB1 sends a SN STATUS TRANSFER message tothe eNB3. The message includes the communication status informationregarding the radio terminal 4 on the cell 10. The steps S1711 to S1712of FIG. 22 are similar to steps S1512 to S1513 of FIG. 20. In step S1713of FIG. 22, the eNB3 and the UE 4 resume the service A of the PCell inthe cell 30. In step S1714, the eNB3 sends to the eNB2 an instruction(Resource Resuming Request) to resume the service B of the SCell. Instep S1715, the eNB2 and the UE 4 resume the service B of the SCell inthe cell 20.

The instruction (Service Resuming Request) to resume the service B instep S1714 of FIG. 22 may be transmitted by the eNB1 or the UE 4 insteadof the eNB3. In this case, the eNB1, the eNB2, or the UE 4 may furthersend a notification of resuming the service B to the eNB3.Alternatively, the eNB1 may send a preliminary notification to the eNB3to inform that the eNB2 is providing the service B in the cell 20. Thispreliminary notification may be transmitted using a handover preparationrequest, a SN status transfer message, or a new message.

In steps S1712 to S1714 of FIG. 22, the following processes may befurther performed. After the eNB1 receives a handover completionnotification (UE context release) and the handover (i.e., the change ofthe PCell) is completed, the eNB1 sends to the eNB2 an instruction toestablish a connection with the eNB3. In response to the connectionestablishment instruction from the eNB1, the eNB2 sends a connectionestablishment request to the eNB3. The eNB2 and the eNB3 then establisha connection. After that, the eNB3 sends to the eNB2 an instruction(Service Resuming Request) to resume the service B. The establishment ofconnection between the eNB2 and eNB3 includes establishment of an X2interface between eNBs, establishment of connection through an S1interface and the EPC, or establishment of a new interface (e.g., X3)between an eNB and an LPN, for example.

Procedure Example 7

In Procedure Example 7, the process of the EPC 5 is added to ProcedureExample 6. FIG. 23 illustrates an example of a sequence diagramillustrating the entire process of Procedure Example 7. In FIG. 23, thecells 10, 20, and 30 are denoted by CELL1, CELL2, and CELL3,respectively. The processes of steps S1801 to S1809 of FIG. 23 are thesame as the processes of steps S1701 to S1710 of FIG. 22. In step S11810of FIG. 23, the eNB3 sends, to the EPC 5 (e.g., a Mobility ManagementEntity (MME)), a bearer switch request for switching the bearer of theradio terminal 4, which passes through the cell 10 of the eNB1, to abearer through the cell 30 of the eNB3 (Path switch of bearer ateNB1/Cell1 to eNB3/Cell3). The processes of steps S1811 to S1814 of FIG.23 are similar to the processes of steps S1712 to S1715 of FIG. 22.

Other Embodiments

The first and second embodiments described above may be applied to C/USplit configuration in which a macro cell having a wide coverage is usedfor transmission and reception of control-plane signals (C-Planesignals) such as mobility management of a UE and a pico cell whichprovides relatively better communication quality is used fortransmission and reception of data-plane signals (U-Plane signals) suchas user data. For example, the cells 10 and 30 of the eNB1 and eNB3 maybe used for transmission and reception of C-Plane signals and the cell20 of the eNB2 may be used for transmission of U-Plane signals.

The first and second embodiments described above can be applied to whenchanging the primary cell (PCell) to another cell of the radio station(eNB) 1 (e.g., a cell using a different frequency or arranged in adifferent geographical area). Moreover, the first and second embodimentscan be applied to a configuration in which a plurality of SCells ispresent.

The first and second embodiments described above can be applied to aconfiguration in which the PCell and SCell employ different duplexmodes. For example, one of the PCell and SCell may employ a frequencydivision duplex (FDD) and the other may employ a time division duplex(TDD).

All the communication control methods performed by the radio station 1(communication control unit 15), the radio station 2 (communicationcontrol unit 25), the radio station 3 (communication control unit 35),and the radio terminal 4 (communication control unit 45) described inthe first and second embodiments may be implemented by using asemiconductor processing device including an Application SpecificIntegrated Circuit (ASIC). Alternatively, these methods may beimplemented by causing a computer system including at least oneprocessor (e.g., microprocessor, Micro Processing Unit (MPU), DigitalSignal Processor (DSP)) to execute a program. Specifically, one or moreprograms including instructions for causing a computer system to performthe algorithms shown in the flowcharts and the sequence diagrams may becreated and these programs may be supplied to a computer.

These programs can be stored and provided to a computer using any typeof non-transitory computer readable media. Non-transitory computerreadable media include any type of tangible storage media. Examples ofnon-transitory computer readable media include magnetic storage media(such as flexible disks, magnetic tapes, hard disk drives, etc.),optical magnetic storage media (e.g., magneto-optical disks), CompactDisc Read Only Memory (CD-ROM), CD-R, CD-R/W, and semiconductor memories(such as mask ROM, Programmable ROM (PROM), Erasable PROM (EPROM), flashROM, Random Access Memory (RAM), etc.). These programs may be providedto a computer using any type of transitory computer readable media.Examples of transitory computer readable media include electric signals,optical signals, and electromagnetic waves. Transitory computer readablemedia can provide a program to a computer via a wired communication line(e.g., electric wires, and optical fibers) or a wireless communicationline.

In the above first and second embodiments, the LTE system has beenmainly described. However, these embodiments may be applied to radiocommunication systems other than the LTE system, for example, a 3GPPUniversal Mobile Telecommunications System (UMTS), a 3GPP2 CDMA2000system (1×RTT, High Rate Packet Data (HRPD)), a Global System for MobileCommunications (GSM) system, or a WiMAX system.

Further, the above embodiments are merely examples of applications oftechnical ideas obtained by the present inventors. Needless to say,these technical ideas are not limited to the above embodiments and maybe changed in various ways.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-288211, filed on Dec. 28, 2012, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   1 FIRST RADIO STATION-   2 SECOND RADIO STATION-   3 THIRD RADIO STATION-   4 RADIO TERMINAL-   5 CORE NETWORK-   6 RADIO NETWORK-   10 FIRST CELL-   20 SECOND CELL-   30 THIRD CELL-   15 COMMUNICATION CONTROL UNIT-   25 COMMUNICATION CONTROL UNIT-   35 COMMUNICATION CONTROL UNIT-   45 COMMUNICATION CONTROL UNIT

1. A method in a radio terminal, the method comprising: performing DualConnectivity by using a first cell served by a first radio station as aprimary cell and a second cell served by a second radio station as asecondary cell; receiving a message from the first radio station tochange the primary cell from the first cell to a third cell operated bya third radio station; in a case where the radio terminal changes theprimary cell from the first cell to a third cell served by a third radiostation, reconfiguring at least one of a Packet Data ConvergenceProtocol (PDCP) layer or a Radio Link Control (RLC) layer correspondingto a data radio bearer (DRB) established in the second cell in responseto the message; and resuming communication with the second radio stationin the second cell using the DRB established in the second cell afterthe primary cell is changed.
 2. A method in a first radio station,comprising: communicating with a radio terminal that performs DualConnectivity by using a first cell served by the first radio station asa primary cell and a second cell served by a second radio station as thesecondary cell; and instructing, by transmitting a message for changingthe primary cell from the first cell to a third cell served by a thirdradio station, the radio terminal to reconfigure at least one of aPacket Data Convergence Protocol (PDCP) layer or a Radio Link Control(RLC) layer corresponding to a data radio bearer (DRB) established inthe second cell and to resume communication with the second radiostation in the second cell using the DRB established in the second cellafter the primary cell is changed.
 3. A radio terminal comprising: amemory storing instructions; and at least one processor configured toprocess the instructions to: perform Dual Connectivity by using a firstcell served by a first radio station as a primary cell and a second cellserved by a second radio station as a secondary cell; receive a messagefrom the first radio station to change the primary cell from the firstcell to a third cell operated by a third radio station; in a case wherethe radio terminal changes the primary cell from the first cell to athird cell served by a third radio station, reconfigure at least one ofa Packet Data Convergence Protocol (PDCP) layer or a Radio Link Control(RLC) layer corresponding to a data radio bearer (DRB) established inthe second cell in response to the message; and resume communicationwith the second radio station in the second cell using the DRBestablished in the second cell after the primary cell is changed.
 4. Afirst radio station, comprising: a memory storing instructions; and atleast one processor configured to process the instructions to:communicate with a radio terminal that performs Dual Connectivity byusing a first cell served by the first radio station as a primary celland a second cell served by a second radio station as the secondarycell; and instruct, by transmitting a message for changing the primarycell from the first cell to a third cell served by a third radiostation, the radio terminal to reconfigure at least one of a Packet DataConvergence Protocol (PDCP) layer or a Radio Link Control (RLC) layercorresponding to a data radio bearer (DRB) established in the secondcell and to resume communication with the second radio station in thesecond cell using the DRB established in the second cell after theprimary cell is changed.
 5. A radio communication system comprising:first, second, and third radio stations configured to respectively servefirst, second, and third cells; and a radio terminal configured toperform Dual Connectivity by using the first cell as a primary cell andthe second cell as a secondary cell, wherein the first radio stationcomprises: a memory storing instructions; and at least one processorconfigured to process the instructions to transmit, to the radioterminal, a message for changing the primary cell from the first cell tothe third cell, and the radio terminal comprises: a memory storinginstructions; and at least one processor configured to process theinstructions to: receive a message from the first radio station tochange the primary cell from the first cell to a third cell operated bya third radio station; in a case where the radio terminal changes theprimary cell from the first cell to a third cell served by a third radiostation, reconfigure at least one of a Packet Data Convergence Protocol(PDCP) layer or a Radio Link Control (RLC) layer corresponding to a dataradio bearer (DRB) established in the second cell in response to themessage; and resume communication with the second radio station in thesecond cell using the DRB established in the second cell after theprimary cell is changed.